Current Gigabit/s Passive Optical Networks (GPONs) have a standard operating speed of 2.5 Gb/s. However, standards for GPONs having an operating speed of 10 Gb/s are under development which will require an upgrade of existing networks. A straightforward approach to upgrade the network is to upgrade the Optical Line Termination (OLT) from 2.5 Gb/s to 10 Gb/s and then upgrade the Optical Network Termination (ONT), sometimes referred to as an Optical Network Unit (ONU), of every subscriber. However, such an approach is likely to be prohibitively expensive. Further, the network will experience downtime until all of the subscribers are upgraded, which may be a couple of days to a couple of weeks time period.
One alternative upgrade scenario would be to upgrade the OLT to 10 Gb/s first, but provide backwards compatibility such that legacy ONTs can interoperate with the new OLT. Thus the new OLT must also be configured to run at 2.5 Gb/s (legacy speed). Such a system is considered to be 10-Gb/s-ready, but is still not running at 10 Gb/s. The upgrades of the ONTs will occur only when the subscriber requests them, with the understanding that some subscribers may never request to upgrade and thus will remain with equipment configured for 2.5 Gb/s only.
Thus at some stage during the upgrade, the PON system would consist of a dual-speed OLT (i.e., an OLT capable of transmitting at either 10 Gb/s or 2.5 Gb/s) and a mixture of advanced and legacy subscribers (i.e. subscribers whose ONT runs at 10 Gb/s and subscribers whose ONT runs at 2.5 Gb/s). In such a network, the dual speed OLT may be transmitting to either legacy or advanced ONT switching the speed from one to the other.
To provide dual speed transmission, a frame format is used having consecutive 10 G and 2.5 G partitions. A problem with dual speed transmission is that during a 10 G partition for example, legacy ONTs are not receiving data but must maintain their Phase-Locked Loops (PLL)s in a locked state, i.e. they must keep the recovered clock stable and unchanged, so that they can receive the data when the 2.5 G partition starts. During the 2.5 G partition, legacy ONTs are receiving data, while advanced ONTs are maintaining the recovered clock locked. If both partitions use the most commonly used non-return to Zero on-off keying (NRZ-OOK) modulation format, maintaining the clock locked by the legacy receivers during the 10 G partition would not be possible due to inherent phase ambiguity that exists in every signal that is an integer times faster than the nominal signal.
What is required is a system, method and computer readable medium capable of providing dual speed transmission in a passive optical network.